1. Technical Field
This invention relates to coil arrangements and in particular to coil arrangements for use in Magnetic Resonance Imaging and Spectroscopy (MRIS).
2. Related Art
MRIS systems generally comprise a plurality of coils which are located around a region within which a patient or some such other subject can be positioned. The coils include an outermost DC coil arrangement, an inner RF coil arrangement positioned concentrically therewithin and an intermediate gradient coil assembly. The DC coil arrangement is used to provide a strong constant magnetic field. The gradient coil assembly is arranged to generate a time-varying magnetic field which causes the response frequency of the nuclei of the subject to depend upon their positions within the field.
Gradient coil assemblies usually consist of, in a minimum configuration, three primary field-generating coils, referred to herein as the X, Y and Z drive coils. They can be formed by winding a pattern of conductors on the surface of cylinder. Commonly, each of the coils is shielded by another pattern of conductors, termed “shield coils”, wound on another cylinder which surrounds the drive coils. The drive coils are shielded to avoid inducing eddy currents in surrounding metallic structure and consequential time-varying magnetic fields in the subject.
Conventional X and Y drive coils each typically comprise four coils, arranged into two pairs, one pair being radially opposite the other and the coils in each pair being axially juxtaposed and aligned with respect to the cylinder. Each coil extends angularly around the surface of the cylinder slightly less than 180°. The appearance of the coils is such that they are termed “saddle windings”. The four coils that make up the X drive coils are connected in series and the four coils that make up the Y drive coils are connected in series. A conventional arrangement whereby the coils are connected in series is shown in FIG. 1. The arrangement shown in FIG. 1 is shown unwrapped from a cylindrical surface. For each of the X and the Y drive coils, the coils of each opposite pair are connected together at the centres thereof, and the outside of one coil of one pair is connected to the outside of one coil of the other pair. The outside of the other coil of the one pair and the outside of the other coil of the other pair are each connected to a respective terminal of a power supply unit (not shown). X and Y drive coils are substantially similar to one another in design, with X drive coils being positioned radially inside or outside Y drive coils and angularly displaced by 90 degrees from the Y drive coils, with respect to the cylinder.
Drive coils are typically impregnated with epoxy resin to provide electrical insulation and mechanical strength. However, if a potential difference above a certain threshold exists between spatially adjacent resin-impregnated drive coils, local charge redistribution can occur within microscopic voids and inhomogeneities in the resin. This phenomenon is termed “partial discharge” and can create broad-band electrical interference that is deleterious to the sensitive radio frequency detection systems used in MRIS.